A standard headrest such as used in a motor vehicle comprises a generally stationary support typically formed by a seat back or rear shelf, a headrest body having a forwardly directed front face and positioned such that a head of a user seated in front of the support is generally level with the body, and a guide securing the body to the support for vertical movement relative to the support. The primary function of such a headrest is to protect the user against a whiplash injury that is caused in an collision when the user's head is snapped back suddenly. The headrest prevents excessive backward displacement of the user's head so as to prevent or mitigate any injury that can be caused by overextension of the cervical vertebrae.
For maximum protection the headrest must be positioned so that it solidly engages the center of the back of the user's head. For esthetic and other reasons it is not practical to make the headrest so large that it will function for user's of all sizes, so the above-mentioned vertical adjustability is used to ensure proper positioning. Of course this depends on the headrest actually being adjusted, and practice has shown that users often do not bother to adjust it at all or only roughly set it so that it does not function as needed.
Accordingly it is known to provide a drive for vertically shifting the body relative to the support, some sort of sensor for detecting the vertical position of the user's head, and a controller connected to the sensor and to the drive for displacing headrest body the body so as to centrally align it with the user's head. Typically the headrest is connected to the upper seat back of the vehicle seat via two support rods, and the drive is mounted in the seat back so the headrest with the two support rods can be moved relative to the fixed vehicle seat. Alternatively there is the option for providing the drive in the headrest, in which case the support rods are securely attached to the upper seat back and it is possible to move the headrest along to the fixed support rods . . . .
Thus for example DE 199 33 769 of H-M. Lent-Philipps provides for adjustment of the interior mirror and an angular adjustment of the seat back of the vehicle seat. In such sensor assemblies there is fundamentally the risk that incorrect adjustments, e.g. incorrect adjustments of the interior mirror, will not be recognized by the sensor assembly and thus the headrest will be set incorrectly, as well.
In another known sensor assemblies, such suggested in DE 198 17 199 of W. Buss, transmitters and receivers use for instance ultrasonic signals or infrared signals in order to determine the position of a passenger's head. Other headrests, for instance in accordance with U.S. Pat. No. 4,465,233 of K. Bruse, also suggest light transmitters and receivers. Such sensor assemblies that work with transmitters and receivers of electromagnetic waves and that attempt to measure the reflection of the electromagnetic waves from the passenger's head are termed surface-sensitive sensor assemblies. Problems occur in these cases when the passenger is wearing a head covering, e.g. a hat, or when the passenger is reading a newspaper, because the electromagnetic waves are reflected on any surface, that is also on the head covering or on the newspaper. Thus this can also lead to incorrect positioning of the headrest.
Finally, headrests have also become known that for instance in accordance with DE 100 65 845 of P. Schumann provide contact lines, which require, however, that the passenger's head be positioned against the headrest in order to be able to detect the head position. In U.S. Pat. No. 7,145,263 of Nathan a capacitive sensor is used as the headrest sensor. How such a sensor is embodied cannot be found in this publication, nor does it provide any indication about how a specific embodiment of the measurement electronics or control electronics could be undertaken.